Traffic Graph out shows zero for ath0_wlan0 device

Bug reported: https://redmine.pfsense.org/issues/4028
Looking at traffic graph on a WiFi card installed in an Alix 2D13 - ath0_wlan0
It is not a super-important thing, but there might be other device name combinations that have this issue also, or other interesting or more important cases where pfSense_get_interface_stats() or something underneath that is not quite right.